The present invention relates to a disordered fluorite-type crystal containing Sm.sup.2+ as active ions, which crystal is a photochemical hole burning material suitable for high-density recording.
Photochemical hole burning (PHB) materials are now drawing much attention as a recording device which enables far higher recording density than currently available optical disks.
A PHB material is formed by dispersing photoactive ions in a proper matrix. Since photoactive ions occupy various sites of a crystal, a light absorption spectrum of the crystal has a non-uniform band. When laser light of a narrower wavelength range than the absorption band is applied to the crystal, only the photoactive ions resonating with the laser light are excited. Therefore, if part of the photoactive ions are rendered inactive by irradiation with laser light (writing operation), a steep drop (i.e., a "hole") appears in an absorption spectrum (reading operation). Ultrahigh-density recording can be realized by associating the presence/absence of a hole with digital information.
In the beginning of the development, photochemical PHB materials were formed by doping an inorganic polymer or a crystal with organic molecules capable of effecting the hole burning.
Recently, it has been proposed to dope a crystal with rare-earth ions. For example, it has been reported that a hole is created in BaFCl.sub.0.5 Br.sub.0.5 :Sm.sup.2+ crystal at the liquid nitrogen temperature (C. Wei, S.Haung, et al., J. Luminescence, Vol. 143, pp. 161 (1989)). However, this crystal requires a very low operating temperature to effect hole burning. At a high operating temperature, an increased hole width reduces the recording density of a resulting recording device.
It has been reported that hole burning is observed at the room temperature in SrFCl.sub.0.5 Br.sub.0.5 :Sm.sup.2+ (R. Jaaniso and H. Bill, Europhys. Lett., Vol. 16, pp. 569 (1991)). However, this crystal is produced only in a small size of at most 400 .mu.m. Therefore, it cannot be said that this crystal has sufficient performance in practical use particularly in easiness of manufacture and material size. In addition, this crystal is considered to have another problem in practical use, i.e., a hole filling phenomenon in which a considerable part of a first hole is filled when a second hole is created adjacent to the first hole.
On the other hand, it has been reported that hole burning is observed at the room temperature in non-oxide glass, in particular, fluoride glass containing rare-earth ions, such as HfF.sub.4 and BaF.sub.3 (K. Hirao, S. Todoroki, et al., J. Non-Cryst. Solids, Vol. 152, pp. 267). However, because of a wide hole created, this type of materials are not suitable for practical use.
It has also been reported that oxide glass, more specifically, borate glass containing bivalent Sm ions is a stable burning material at the room temperature (K. Hirao, S. Todoroki, et al., Opt. Lett., Vol. 18, pp. 1586 (1993). Although this material can provide a smaller hole width than the above non-oxide materials, it is still not suitable for practical use.
At the present time, there has not been found any bulk single crystal material in which stable hole burning occurs at a temperature higher than the liquid nitrogen temperature, in particular, at the room temperature to create a sufficiently narrow hole, and which does not suffer from the hole filling phenomenon.